1. Field of the Invention
The invention relates to bolted flange connections and, particularly, to such connections for limiting transmitted loads during unbalance events in turbomachinery.
2. Description of Related Art
Flanges are generally held together by bolts through bolt holes provided in each of the flanges and that are aligned with each other. Large radial, tangential, or axial loads with respect to an axial centerline of the bolted joint can impose bending moments or tensile forces in the flange that can cause deformation or rupture of the bolts.
During gas turbine engine operation, a foreign body, such as a bird, could impact the fan assembly and cause part or all of a fan blade to become detached from the rotor disk. Such blade loss is a certification requirement under FAA 14 CFR part 33 rules. Fan blade loss creates a large rotor imbalance, particularly, in early revolutions of the imbalance causing event. This results in the transmission of potentially damaging imbalance forces to the bolted connection, possibly resulting in the misalignment of the flanges and bearing which they help support.
This could be a particular problem in gas turbine engines and, more so, in aircraft gas turbine engines that use bolted connections to support bearings which, in turn, support rotatable rotors. Rotor blade failures, which can be caused by foreign objects that are drawn into the fan or compressor, can cause rotor unbalance conditions. Such rotor unbalance conditions can impose radial, circumferential, and possibly also axial loads sufficient to fail supporting structure causing loss of centerline. Such rotor unbalance conditions can cause unintended high shear, bending, or tensile loads, or a combination of such loads, applied to the flange connecting bolts, leading to structural damage, bolt deformation, and possibly to bolt rupture and separation of the bolted casings from each other. This can cause the centerline of the flange of the bearing casing to shift. This, in turn, can shift the radial location of the bearings which is an undesirable condition even with the engine shutoff and windmilling. Thus, it is highly desirable to maintain the centerline of the flange of the bearing casing supporting the bearing when there has been an unbalance load event like fan blade out and the engine will be shutoff and the fan windmilled.